1. Field of the Invention
The present invention relates to mold making, and particularly to a method and apparatus for preparing a pre-investment pattern. More specifically, the present invention is related to the making of vane patterns for use in investment casting of annular stator vane rings, for instance, such as are used in gas turbine engines.
2. Description of the Prior Art
In the production development of any new gas turbine engine or in modification of any existing production engine, it is often necessary, in order to obtain optimum efficiency, to adapt the turbine nozzle section, that is, the throat area of the turbine stator vane ring to the prevailing conditions encountered when the engine is completed and tested. This requires modification of the vane stagger angle, shroud or vane spacing of each turbine stator due to manufacturing tolerances of the compressor, diffuser, etc. The throat area can be adjusted to a certain extent by casting excess metal in the nozzle sections between the vanes to allow for trimming. However, this allows adjustment only in one direction, that is, to increase the throat area. It also removes any protective coating. Another manner of modifying the throat area is to bend the airfoils of the vanes, but this can result in performance losses and/or turbine excitation problems. All of these suggested modifications are based on the correct assumption that the vanes are made by investment casting methods and that to modify the throat area of the vane ring by making pattern changes is prohibitively expensive and time consuming since the tooling to make the molds for the pre-investment patterns must be changed and so on.